A method and device for providing electrical stimulation to a patient has previously been described in our international patent applications WO 2006/054118, WO 2010/070332 and WO 2014/001778. The contents of these publications are incorporated herein by reference.
When directly stimulating a nerve with superficial electrodes, it is well established in conventional Functional Electrical Stimulation that the current required to provide effective stimulation depends on the positioning of the electrodes relative to the nerve. The closer that the direct current path between electrodes passes to the nerve motor point in question, the less current is required to stimulate the nerve. Similarly, a given current is more effective in eliciting a muscle contraction if it passes closer to the specific motor nerve point. Therefore, the effectiveness of a nerve stimulation device comprising a simple pair of electrodes is highly dependent on the correct positioning of the electrodes. Consequently, in neuromuscular stimulation devices, it is necessary to apply an electrode to a suitable place on the body to stimulate a nerve. Frequently this position is not optimal in the first instance, and repeated attempts are required to position the electrode in the best position. This also applies to other types of stimulation, and to other applications for electrodes, including recording modes such as electromyography (EMG).
The optimal position for an effective electrode for stimulation of a particular nerve can be determined by sequential activation of subsets of electrodes chosen from an array. This allows the site of stimulation to be moved electronically rather than physically in relation to the nerve. The subsets of activated electrodes form distinct “virtual electrodes” or effective electrodes. Within such an electrode array, each electrode may be paired with its counterpart in an adjacent row in sequence. The direct current path between the activated electrodes, and hence the effective electrode, can thereby be moved incrementally as different pairs of electrodes are addressed. This allows for sequential adjustment of the position of the effective electrode, enabling the device to scan across a region, allowing the current path to pass very close to the optimum stimulation point, wherein the optimum pair of electrodes is addressed. This makes the positioning of the device relative to the nerve, for example on the skin, much less critical.
Heller et al. Medical Engineering & Physics 35 (2013), pages 74-81, describes a method for electrical stimulation of the leg in order to correct foot drop. Stimulation is delivered via a 4×4 subset of electrodes, chosen automatically according to a computer algorithm from an 8×8 array.
The applicant's previous patent application WO 2006/054118 describes an array of electrodes on a single substrate, allowing adjustment and selection of different positions of the effective electrode without removal or repositioning of the assembly.
International patent application WO 2007/015907 relates to a device comprising an electrode array for implanting into the tissue, in particular cardiac tissue. Different electrode subsets from the array can be activated in order to produce localised electrical fields.
US patent application US 2010/0076521 relates to a device comprising an electrical stimulation system for generating virtual channels for transferring external signals to nerve fibres and then the brain, for example for the purpose of restoring patients' vision or hearing. The virtual channels result from the overlap of the electrical fields of the individual electrodes within an electrical array, and thereby improve the resolution of neural stimulation.
The limitation of the array approach described in the above referenced publications is that, for a given electrode size, adjustments to the effective electrode position may only be made in whole units of the electrode, for example if the electrode is 1 cm×1 cm, the effective electrode position can only be adjusted in minimum increments of 1 cm.
Korean Patent Application KR 20120143273 describes an artificial cochlea for stimulating the auditory nerve. The electrodes of the device are positioned on conductive tracks surrounded by insulating layers. The insulating layers vary in size around the electrodes and the conductive tracks between the electrodes, such that the area of the conductive track between the electrodes is narrower than the area of the conductive track around the electrode. The arrangement allows for flexible positioning of the effective electrodes and thereby the electrical stimulus. As with an electrode array, the ability to adjust the position of the electrode may only be made in whole units of the electrode size formed by the conductive tracks.
When searching for the correct stimulation position, it would be advantageous to adjust the position of the effective electrode of a stimulation device in steps smaller than the electrode size. The present invention relates to electrode geometries which allow this.